1. Field of the Invention
The present invention relates to thin-film semiconductor devices for driving active-matrix liquid crystal displays and organic electroluminescence (EL) displays, and in particular to a structure and a manufacturing method of a thin-film semiconductor device for display apparatus which device has a dual-channel-layer structure in which a polycrystalline semiconductor channel layer and a non-crystalline semiconductor channel layer are provided.
2. Description of the Related Art
In recent years, as one of the next-generation flat panel displays replacing liquid crystal displays, attentions have been focused on an organic electroluminescence (EL) display using electroluminescence of an organic material. Organic EL displays are current drive devices different from liquid crystal displays that are of a voltage drive type. There is therefore a pressing necessity to develop a thin-film semiconductor device having excellent on-off characteristics as a drive circuit for active-matrix display apparatuses.
As a thin-film semiconductor device in a drive circuit for liquid crystal displays, there is conventionally a thin-film semiconductor device in which a channel layer is a single layer made of a non-crystalline semiconductor. The thin-film semiconductor device of this type has a wide band gap and therefore flows a small amount of off-state current, but involves a problem of on-state current being also small due to low mobility.
There is also a thin-film semiconductor device in which a channel layer is a single layer made of a polycrystalline semiconductor, as a thin-film semiconductor device in a drive circuit for a liquid crystal display. The thin-film semiconductor device of this type has high mobility, thus flowing a large amount of on-state current, but involves a problem of off-state current being also high due to the presence of grain boundaries and defects in the polycrystalline semiconductor, in contrast to the thin-film semiconductor device in which a channel layer is a single layer made of a non-crystalline semiconductor.
To solve these problems, a thin-film semiconductor device has been proposed which has a dual-channel-layer structure in which a polycrystalline semiconductor channel layer and a non-crystalline semiconductor channel layer are provided (refer to Hatzopoulos et al., IEEE ELECTRON DEVICE LETTERS 28, 803 (2007), for example). It is said that providing the channel layer with the dual-layer structure having a polycrystalline semiconductor channel layer and a non-crystalline semiconductor channel layer enables interaction of advantages of the respective layers, thereby ideally resulting in such characteristics that the on-state current is higher than that in the case of a non-crystalline semiconductor single-channel layer and the off-state current is lower than that in the case of a polycrystalline semiconductor single-channel layer.
However, in an actual thin-film semiconductor device, providing the channel layer with the dual-layer structure having a polycrystalline semiconductor channel layer and a non-crystalline semiconductor channel layer causes interaction of defects of the respective layers, therefore not necessarily being capable of increasing the on-state current and decreasing the off-state current. That is, in a front channel which, as a pathway for charges, runs through both of the non-crystalline semiconductor channel layer and the polycrystalline semiconductor channel layer between source and drain electrodes, the charges move across the non-crystalline semiconductor channel layer having high resistance, which causes a problem of decreasing on-state current. Furthermore, when, as a pathway through which charges move, not only a front channel running through both of the non-crystalline semiconductor channel layer and the polycrystalline semiconductor channel layer, but also a back channel running through only the non-crystalline semiconductor channel layer, are present between the source and drain electrodes, the non-crystalline semiconductor channel layer acts as a parasitic current pathway, which increases off-state current as leak current.
As a disclosed technique of increasing on-state current in order to deal with these problems, there is a disclosure of a thin-film semiconductor device including a channel layer having a protruding shape (refer to U.S. Pat. No. 6,794,682, for example). According to this disclosed technique, when, in a part below the protruding shape of the channel layer which part serves as a current pathway, current flows between source and drain electrodes through the parts of the channel layer below both sides of the protruding shape, a resistance component in a vertical direction of the channel layer can be smaller because the parts of the channel layer below the both sides of the protruding shape of the channel layer are thinner than a part of the protruding shape of the channel layer. This makes it possible to keep the resistance across the part below the protruding shape of the channel layer low and to thereby increase on-state current. The upper part of the protruding shape of the channel layer acts as a resistance between the source electrode and the drain electrode. Charge movement is therefore suppressed in the back channel between the source electrode and the drain electrode.
On the other hand, as a disclosed technique of decreasing off-state current, there is a technique that, in a thin-film semiconductor device having a dual-layer structure in which a polycrystalline semiconductor channel layer and a non-crystalline semiconductor channel layer are provided, the non-crystalline semiconductor channel layer has a recess formed by over-etching, and inside the recess, a region of a conductivity type opposite to a conductivity type of a contact layer is formed (refer to Japanese Unexamined Patent Application Publication No. 2009-060096, for example). This disclosed technique has an advantage of suppressed charge movement in the recess formed in the non-crystalline channel layer because the recess is of the conductivity type opposite to the contact layer.